Impregnated diamond earth-boring bits may be used for drilling hard or abrasive rock formations such as sandstones. Typically, an impregnated diamond bit has a solid head or crown that is cast in a mold. The crown is attached to a steel shank that has a threaded end that may be used to attach the crown and steel shank to a drill string. The crown may have a variety of configurations and generally includes a cutting face comprising a plurality of cutting structures, which may comprise at least one of cutting segments, posts, and blades. The posts and blades may be integrally formed with the crown in the mold, or they may be separately formed and attached to the crown. Channels separate the posts and blades to allow drilling fluid to flow over the face of the bit.
Impregnated diamond bits may be formed such that the cutting face of the drill bit (including the posts and blades) comprises a particle-matrix composite material that includes diamond particles dispersed throughout a matrix material. The matrix material itself may comprise a particle-matrix composite material, such as particles of tungsten carbide, dispersed throughout a metal matrix material, such as a copper-based alloy.
While drilling with an impregnated diamond bit, the matrix material surrounding the diamond particles wears at a faster rate than do the diamond particles. As the matrix material surrounding the diamonds on the surface of the bit wears away, the exposure of the diamonds at the surface gradually increases until the diamonds eventually fall away. As some diamonds are falling away, others that were previously buried become exposed, such that fresh sharp diamonds are continuously being exposed and used to cut the earth formation being drilled.
Typically, an impregnated diamond bit is formed by mixing and distributing diamond particles and other hard particles, such as, particles of tungsten carbide, in a mold cavity having a shape corresponding to the bit to be formed. The diamond particles and hard particles are then infiltrated with a molten metal matrix material such as, for example, a copper-based metal alloy. After infiltration, the molten metal matrix material is allowed to cool and solidify. The resulting impregnated diamond bit may then be removed from the mold. Alternatively, a mixture of diamond particles, hard particles, and powder matrix material may be pressed into the a desired shape to form a green bit body, and the green bit body may then be sintered one or more times to form an impregnated diamond bit having a desired final density.
During such fabrication processes, the diamond particles may not be uniformly dispersed throughout the matrix material. The diamond particles have a tendency to agglomerate together, leaving a greater density of diamonds in some regions of the bit relative to other regions of the bit. This may result in two or more diamond particles lying in contact with one another rather than being uniformly dispersed, as desired. These diamond-to-diamond contacts may substantially weaken the impregnated diamond bit and may result in uneven drilling and chipping or fracture of the blade or post on the bit.